An impact driver is known from granted Japanese patent no. 4981345 that uses a motor for rotating a spindle via a speed reducing planetary gear mechanism. The rotational force of the motor is converted to rotational impact force via a hammer peripherally provided on a front end part of the spindle. The hammer is mounted so that it is urged frontward by a compression spring (i.e., a spring).
Such a device includes a pin that passes through a rear part of the spindle and that serves as a rotary shaft of a planetary gear of the planetary gear mechanism. In order to retain this pin, a washer is provided on the front side of the rear part of the spindle that presses the pin rearward. This washer receives the spring on its front side and is shaped such that the immediate inner side of the portion that receives the spring bulges frontward in order to properly position and/or prevent mispositioning of the spring.